The present invention relates generally to pultruded members and in particular to tubular pultruded members having uniform wall thickness as well as methods of making such tubular pultruded members.
The pultrusion process generally consists of pulling continuous lines of fibers through a resin bath or impregnator and then into a preforming fixture where the section is partially shaped and excess resin and/or air are removed and then into heated dies where the section is cured continuously. Typically, the process is used to make fiber glass reinforced pultruded shapes. For a detailed discussion of pultrusion technology, reference is directed to "Handbook of Pultrusion Technology" by Raymond W. Meyer, first published 1985 by Chapman & Hall, New York which is hereby incorporated by reference in its entirety herein. In the pultrusion process the fibers are submersed in a polymer bath and drawn through a die opening of suitable shape and high temperature to produce a solid piece of dimensions and shapes of the die which can be cut, shaped and machined. Typically, holes, slots, ridges, grooves, contact areas or screw threads may be formed in the pultruded composite member by conventional machining techniques. Alternatively, the pultrusion process maybe modified such that when the pultrusion is initially removed from the die it is pliable and can be bent or otherwise shaped to a form which upon further curing becomes a rigid structural member. Further, if the resin is a thermoplastic the process can be adjusted such that the part is removed hot from the die, shaped and then cooled to solidify or subsequently heated, formed and then cooled.
In making tubular or hollow pultruded members a mandrel is installed to span from ahead of the polymer bath through the forming die section in the heated die and often totally through the heated die itself. The internal hole feature of the tubular pultruded member is created by a cantilevered mandrel of the desired internal shape supported only at the rear portion of the pultrusion machine. As a result, the cantilevered mandrel floats relative to the heated die and due to its length and weight which is unsupported provides variations in wall thickness of the final pultruded member. In addition, since the rovings and mat reinforcement used in the pultrusion manufacturing process may in themselves vary by up to .+-.15% in thickness a further thickness variable in the final pultruded member can be created. Accordingly, the tolerances in the pultrusion process are much greater than those from injection molding processes and are typically of the order of .+-.20% on wall thicknesses for major dimensions less than two inches. By comparison, injection molding parts may have a .+-.2%, or even .+-.0.5% tolerance in thickness. As a result it is difficult if not impossible to use together or interchangeably structural tubular parts made from both an injection molding process and a pultrusion process.
One technique that has previously been used to minimize these difficulties in obtaining uniform wall thickness is to have the pultrusion process run as a vertical process wherein the mandrel is centered more by gravity within the center of the heated die. However, this is a very expensive process, particularly in that it requires an extremely tall building to accommodate the larger pultruded members.